I know some lever rifles with half cock safeties are considered safe so why not a Colt revolver?

If you ever get a chance to compare the hammer and trigger sears of a single action revolver hammer and a lever action rifle hammer you will see the difference right away. The rifle sears are much more substantial.

Also, a handgun hitting the gound is much more likely to be pointing at you than a long rifle would under the same circumstances.

BTW, even the half cock safety on a lever action rifle is not foolproof. They can also break under the right (wrong) circumstances.